Japanese National Patent Publication No. 11-508476 (PTL 1) discloses an oxygenator including a generally cylindrical heat exchanger (of a multipipe type), a blood inlet manifold communicating with a lower end of the heat exchanger, a transition manifold communicating with an upper end of the heat exchanger, a generally cylindrical membrane-type oxygenator concentrically surrounding the heat exchanger and communicating with the transition manifold, and a blood outlet manifold communicating with the membrane-type oxygenator. According to PTL 1, performance as the oxygenator can be enhanced by improving various components constituting the oxygenator.